1. Field of the Invention
The present invention relates to a structure for mounting electronic components and a method for assembling and mounting the electronic components, and more particularly to a structure which includes a housing for mounting electronic components therein and is subjected to a heating process after the electronic components are mounted on the housing.
2. Description of Related Art
Units having this kind of structure include the following electronic modules containing a semiconductor sensor and a controlling IC therein, such as an acceleration sensor module for use in an antilock brake system or an air bag system, a semiconductor pressure sensor module used for detecting suction pressure or atmospheric pressure, a high pressure sensor module used in an air conditioning system, and the like all of the models are used in an automobile. A sensor module of this kind is usually packaged in a metal can having connecting pins for mounting it on a substrate. Mounting the can package on the substrate, however, requires some mounting components, and a structure of the canned module itself tends to be complicated. Therefore, a ceramic package structure has been proposed and used to some extent. Semiconductor sensors and other control elements are on the ceramic package and hermetically sealed therein. The following features are required for surface-mounting components on the ceramic package: securing a bonding strength of bonding wires connecting the components, ensuring reliability of electrical and mechanical connections, obtaining sufficient wettability of solder, and secure reliability of soldered portions.
Considering these requirements, the ceramic package, in which, for example, a tungsten (W) film is formed on a ceramic substrate as a thick film conductor, and, on the tungsten thick film, a nickel-boron (Ni--B) film having a thickness of about 1 .mu.m, a nickel-phosphorus (Ni--P) film having a thickness of about 2 .mu.m and a thick gold (Au) film having a thickness of about 1 .mu.m are plated in this order, has been dominantly used hitherto. Here, the Ni--B plating means a Ni plating in which B is used as a catalyst and the Ni--P plating means a Ni plating in which P is used as a catalyst. Though the film structure of this kind is able to secure the bonding strength and the wettability, reliability of soldering on the Au plating is not secured. This is because the Au atoms diffuse into a solder material during a soldering process, creating voids called Kirkendall voids in the Au film. In addition, the plating cost of the thick Au film is high.
Though, in case a chip condenser is mounted on a package, solder or epoxy-type silver (Ag) paste is customarily used, the solder cannot be used in the ceramic package which is subjected to a heat treatment at a temperature higher than 300.degree. C. to seal the package after the chip condenser is mounted. Also, a silicone-type bonding material is customarily used for die-bonding a sensor chip because a bonding material having low stress characteristics is required for this purpose. There is a problem, however, that bonding materials can not be hardened adequately when the silicone-type material is used together with the epoxy-type material at the same time.